Aneurysmal remodeling in the circle of Willis after carotid occlusion in an experimental model.

Carotid occlusions are associated with de novo intracranial aneurysm formation in clinical case reports, but this phenomenon is not widely studied. We performed bilateral carotid ligation (n=9) in rabbits to simulate carotid occlusion, and sham surgery (n=3) for control. Upon euthanasia (n=3 at 5 days, n=6 at 6 months post ligation, and n=3 at 5 days after sham operation), vascular corrosion casts of the circle of Willis (CoW) were created. Using scanning electron microscopy, we quantified gross morphologic, macroscopic, and microscopic changes on the endocasts and compared findings with histologic data. At 5 days, CoW arteries of ligated animals increased caliber. The posterior communicating artery (PCom) increased length and tortuosity, and the ophthalmic artery (OA) origin presented preaneurysmal bulges. At 6 months, calibers were unchanged from 5 days, PComs further increased tortuosity while presenting segmental dilations, and the OA origin and basilar terminus presented preaneurysmal bulges. This exploratory study provides evidence that flow increase after carotid occlusion produces both compensatory arterial augmentation and pathologic remodeling such as tortuosity and saccular/fusiform aneurysm. Our findings may have considerable clinical implications, as these lesser-known consequences should be considered when managing patients with carotid artery disease or choosing carotid ligation as a therapeutic option.

Improved rat model for cerebral vasospasm studies.

While the rat has been used extensively in subarachnoid hemorrhage (SAH)-cerebral vasospasm studies, concerns exist whether this animal represents a usable model because its time course and pattern of cerebral vasospasm following SAH is not comparable to that observed in man. At present, our knowledge of the rat model is based almost exclusively on studies using a 'single hemorrhage' method. Since there is a positive correlation between severity of cerebral vasospasm, and volume of subarachnoid blood, an obvious question is whether the rat will show modifications in vascular responses when insulted by a second SAH. Here, an SAH was produced in rats using a 'double hemorrhage' method. Following SAH, cerebral arteries showed pathological alterations, significant decreases in luminal perimeter, and increases in arterial wall thickness, over a 7-day post-SAH period. The above vascular features are considered to be indicative of cerebral vasospasm and their presence over a 7-day post-SAH period represents a significant time extension when compared to a single hemorrhage. These modified vascular responses made the double hemorrhaged rat a much-improved animal model.

Formation of the ocular arteries in the chick embryo: observations of corrosion casts by scanning electron microscopy.

An investigation was carried out on the formation of the blood vessels that supply the optic cup or eyeball in developing chick embryos ranging in age from Hamburger-Hamilton stage 17 (st17) to st44. Corrosion casts of the vasculature were made by injecting resin and examined by scanning electron microscopy. The optic cup was supplied by branches of the cranial ramus of the circle of Willis (CCW) from st17 to st19. By st23, a branch of the CCW and that of the internal carotid artery became anlagen of two ophthalmic arteries, namely, the cerebral ophthalmic artery (COA) and the internal carotid ophthalmic artery (ICOA) respectively. They were continued by primordia of the long posterior ciliary arteries, which connected with each other to form a ring around the pupil. Between st19 and st26, another branch of the CCW was found, by contrast, to supply the primitive pecten. The distal part of the nasal long posterior ciliary artery began to atrophy at st28, so the temporal long posterior ciliary artery only began to supply the ring artery around the pupil by st30. At the same time, the artery supplying the pecten became anastomosed with the ICOA behind the eyeball to form the definitive pectinate artery. By st30, the ophthalmic branch of the stapedial artery had also formed and connected with the distal part of the ICOA behind the eyeball, as well as with the distal part of the COA by st34. The supraorbital branch of the stapedial artery, which had replaced the CCW to anastomose with the ethmoidal artery by st30, was found to be connected to the COA at st36. The main vascular system supplying the eyeball was complete at st36, and its structure at st40 and st44 was fundamentally similar to that at st36.

Protective effect of nicardipine treatment on cerebrovascular microanatomical changes in spontaneously hypertensive rats.

1. The effect of long-term treatment with the dihydropyridine Ca2+ antagonist, nicardipine, on the morphology of different sized pial arteries was assessed in spontaneously hypertensive rats (SHR) using histological techniques associated with image analysis. 2. In control 20 week old SHR blood pressure values, the thickness of the tunica media, the media-to-lumen ratio and connective tissue content were significantly increased in comparison with reference normotensive Wistar-Kyoto (WKY) rats. 3. Treatment for 8 weeks with a daily dose of 3 mg/kg of nicardipine decreased blood pressure values in SHR and significantly reduced the area occupied by the tunica media and the media-to-lumen ratio. This effect was observed primarily in small sized pial arteries and to a lesser extent in medium sized pial arteries. Nicardipine administration was without effect on connective tissue content in the wall of cerebral arteries. 4. These results indicate that treatment with nicardipine reduces blood pressure elevation in SHR and exerts a protective effect on arteries controlling cerebrovascular resistance. The activity of the compound primarily on small sized pial arteries may protect the brain from generalized vasodilation which could cause cerebral hypoperfusion.

Distribution of NADPH-diaphorase in the cerebral blood vessels of rats: a histochemical study.

NADPH-diaphorase (neuronal nitric oxide synthase) activity in the cerebral blood vessels was investigated by light and electron microscopic histochemistry to elucidate the sites of nitric oxide production. Networks of adventitial nerves containing NADPH-diaphorase were distributed in all parts of the circle of Willis. However, NADPH-diaphorase activity in adventitial nerves was much sparser in the region of the posterior cerebral artery, and absent in the pial arteries smaller than 100 microns in diameter. Endothelial cells were intensely stained in arteries and arterioles. These results support the hypothesis that vascular tone is regulated by nitric oxide, which is derived from endothelial cells and adventitial nerves.

Evidence against the existence of GABA-B receptor sites in rat cerebrovascular tree.

By the use of combined radioreceptor binding and autoradiographic techniques we attempted to analyse the biochemical characteristics and the anatomical localization of GABA-B receptors in sections of rat circle of Willis and pialarachnoid arteries. 3H-GABA in the presence of 40 microM isoguvacine and 2.5 mM CaCl2 was used to label GABA-B receptor sites. Sections of rat cerebellar cortex were also processed as a reference tissue. No specific 3H-GABA binding was detectable either in radioreceptor binding or autoradiographic experiments. In contrast, the ligand was bound to sections of cerebellar cortex in a manner consistent with the labelling of GABA-B receptor sites. These findings indicate that cerebrovascular physiological responses to GABA are not linked, in the rat cerebrovascular tree, to the activation of GABA-B receptors.

Immunohistochemical localization of an HIV epitope in cerebral aneurysmal arteriopathy in pediatric acquired immunodeficiency syndrome (AIDS).

A 6-year-old boy with acquired immunodeficiency syndrome (AIDS) developed aphasia and quadriplegia 3 months before his death. Cerebral vascular ectasia and multiple cerebral infarcts were noted on premortem radiological studies. Postmortem evaluation revealed diffuse aneurysmal dilatation of the circle of Willis associated with fresh and organizing thrombi, destruction of the elastic lamina, and marked intimal fibroplasia. Multiple cerebral infarcts and subacute AIDS encephalitis with basal ganglia calcification were also present. Immunohistochemistry with a monoclonal antibody (anti-gp41) to human immunodeficiency virus (HIV) demonstrated positively stained cells in the arterial wall of the circle of Willis and in the cerebral parenchyma. Double immunostaining demonstrated that gp41-positive cells in the circle of Willis were also positive for a macrophage marker or leukocyte-common antigen, but not with an endothelial marker. Some macrophages or microglia in the cerebrum were also colabeled with anti-gp41. These results suggest that HIV may be directly involved in vascular pathology associated with pediatric AIDS.

Thalamic arterial pattern: an endocast and scanning electron microscopic study in normotensive male rats.

Rat cerebral vasculature serves as a model for study of the pathophysiology of stroke in humans. Human thalamic arteries show a high incidence of stroke. The objective is to describe the thalamic arterial vascular pattern in normotensive male rats as the initial step for quantitative histochemical studies of enzyme activities in the walls of these vessels. Intracardiac injections of methyl methacrylate monomer provide detailed vascular endocasts. The thalamic vascular bed defined by in situ dissection, serial reconstruction, and light and scanning electron microscopy of endocasts contained four groups of vessel: ventral medial thalamic arteries, thalamic branches from the posterior cerebral artery, and ventral lateral and ventral anterior thalamic arteries. Thalamic vessels are muscular arterioles that, after three to four bipinnate branches, feed into a continuous capillary bed (no loops). The parent vessels and their subsequent branches have been evaluated in terms of their mean internal diameters, mean interbranch intervals, and branch angles. The arterial patterns to rat and human thalami are very similar, with the exception of the anterior choroidal artery which is missing in the rat. The branches supplying the thalamus in both the rat and human are closely associated with the circle of Willis; however, the constituent parts of the circle in rat vary from the pattern in human brain. The rat thalamic arteries show morphological features similar to those seen in the stroke-prone ganglionic arteries in the human basal ganglia.

Cytochemical demonstration of adenylate and guanylate cyclases in vascular smooth muscle of circle of Willis.

The cytochemical localization of adenylate cyclase and guanylate cyclase was studied in the arteries of the circle of Willis in dogs. The reaction products of both adenylate and guanylate cyclases were similarly distributed and selectively localized predominantly adjacent to sarcoplasmic reticulum and sparsely to mitochondria and outer nuclear membranes of vascular smooth muscles. The observations could suggest a close association of the intracellular localizations of both cyclases and the intracellular calcium storage sites, and ultimately contribute to our complete understanding of regulation of cerebral blood flow and vasospasm.

Can the pituitary secrete directly to the brain? (Affirmative anatomical evidence).

Vascular casts of 10 rhesus monkey pituitary glands and three vascular casts of the rhesus monkey cavernous sinus were examined by scanning electron microscopy. A continuous neurohypophyseal capillary bed was found uniting the infundibulum, infundibular stem, and infundibular process. The neurophypophysis was supplied by three groups of arteries: superior hypophyseal, middle hypophyseal, and inferior hypophyseal. Numerous anastomoses were found between individual arteries, and some hypophyseal arteries formed anastomotic links between different portions of the circle of Willis. Veins located at the caudal pole of the infundibular process, capillaries linking the infundibulum to the hypothalamus, and portal vessels extending from the infundibulum to the adenohypophysis provided efferent vascular pathways from the neurohypophysis. The adenohypophysis received no direct arterial supply; its entire afferent vascular supply was provided by portal vessels. Lateral hypophyseal veins were not found; small adenohypophyseal veins joined larger neurohypophyseal veins to form confluent pituitary veins which extended to the cavernous sinus. The capacity of the venous connections draining the adenohypophysis directly to the cavernous sinus appeared small when compared to that of of the long portal vessels supplying the adenohypophysis. However, many of the short portal vessels interposed between the adenohypophysis and the infundibular stem and process were well arranged to function as alternative efferent routes from the adenohypophysis. The limited potential for venous drainage directly to the cavernous sinus suggests that blood leaves the adenohypophysis by other routes; blood carried via long portal vessels from the infundibulum to the adenohypophysis may return to the neurohypophyseal capillary bed via short portal vessels. This anatomical study suggests that hypothalamic and adenohypophyseal secretions are conveyed to the capillary bed of the neurohypohysis. These secretions may leave the neurohypophysis via any of seven potential routes: one efferent route is directed to the adenohypophysis, another route is directed to the systemic circulation, but five of the potential efferent routes are directed toward the brain.

Cell membrane structure of vascular smooth muscle of circle of Willis.

Cell membranes of vascular smooth muscles of the circle of Willis were studied in thin sections and freeze-replicas. The cell membranes were differentiated into a caveolae intracellulares zone and caveolae-free zone, both of which were generally arranged in an alternate manner and parallel to the major axis of the smooth muscle cell. In the former zone, the caveolae intracellulares, about 600 A in diameter, were neatly oriented in one to several rows running parallel to the longitudinal axis of the muscle cell with a center-to center distance of about 800 A. The latter zone was of variable width and smooth, apart from membrane particles or scattered caveolae, and corresponded mainly to the dense area and partially to the myofibril area beneath the cell membrane. Membrane particles were generally more numerous on face A than on face B, and their average number per micronm2 was about twice as many inside the rows of the caveolae as outside. Rosette formations of membrane particles were often evident at the stomal rims of the caveolae. Adherentes and gap junctins were occasionally found on the caveolae-free areas which often protruded externally. Tight junctions appeared as a collection of scattered strands, which frequently showed free ends and were parallel to each other and also to the major axis of the smooth muscle cell.

The prelymphatic pathways of the brain as revealed by cervical lymphatic obstruction and the passage of particles.

Light and electron microscopy was used to examine portions of the brain, the circle of Willis, and the internal carotid arteries of normal cats and rabbits, of sham-operated ones, and of those whose cervical lymphatics had been ligated. Carbon was injected into the cerebral cortex of some lymphoedematous animals. It was found that lymphatic ligation produced oedema of the brain, and a dilatation of the prelymphatic spaces around the vessels. Carbon was traced in these from the injection site, around the minor and major vessels, in the adventitia of the internal carotid artery, entering lymphatics adjacent to it, and finally in the draining lymph nodes. The oedema and dilated spaces were not present in the control animals. This was taken to indicate that there is a continuous system of non-endothelialized spaces and potential spaces-the prelymphatics-draining the brain into the cervical lymphatics. The protein in these spaces appeared to be increased if the lymph-oedema had lasted three weeks as compared to 24 hours, indicating that one of the major roles of this system is the removal of protein.